I received my two driver pairs and tested the RCA Black Plate 12AT7 first. Initially, bass was all over the place, with only 2 hours burn-in, but then it started to gradually become cohesive and synergistic.

The following observations stem from their coupling with RCA 6V6 G ST's (big boys). Consider these preliminary due to insufficient burn-in:

Now bass is more evident and powerful, deeper and tighter. The tube is quite balanced (neutral) and never creates glaring or harshness or sibilance (as the Brimar 12AT7's do sometimes with the big boys).

I also find them smooth, not a hint of edging in extreme frequencies, but not rolled off either, just smooth as silk. A driver that controls the music flow just about right, no excesses, no dullness. No wonder Steve voiced the amp with this driver's specs.

The G ST's do not sound as big/wide with the 12AU7's as they do with SQ 7062's, but this is related to the decreased gain. In no way are the RCA drivers laid back or dull sounding (as my stock 5963's), but they just do not push those big boys so hard. In this respect, I still prefer the SQ 7062's due to its dynamics but it is still to early to tell.

I need to burn them further and then do the same with the GE 6414's, another promising driver alternative. Then, I'll roll each and every one of my output tubes with both new drivers and sort of rank the hot combinations.

My apologies, your message made me realize I had a typo there: It is a pair of RCA 12AU7 Black Plates I'm burning in now, with good results.

Of course, the Phillips SQ 7062 does wonders when coupled with the right output tube, in my case the G ST's.

If the amp sounds too warm and dark given the tube complement and your specific speakers, for example, a good 12AT7 can open up the sound and bring up more transparency, air and neutrality. It has to be the right 12AT7, of course.

I experienced the described effect even using the OA3's (their contribution tends to exacerbate the Reflektor's excessive warmth) along with Russian Reflektor 6V6's in the setup. The latter are quite tough to open up, but when they do, the warmth is just ideal, IMHO. The Brimar 12AT7 pushed the Russian tubes to their best. Unfortunately all other 6V6's in my humble stable do not withstand the Brimars that nicely, with some sibilance or high frequency ringing.

The described setup stood invincible in my system until I tried the 7062's and G ST's together ... Synergy!

I haven't had the time to burn the 12AU7's much more yet and I still need to cook the GE 6414's ... I'll report on both drivers' subjective findings in time.

BTW, congrats on the amazing range of tubes you have had in your tube-rolling experiment. We'll keep an eye for further updates.

No problem. I just need more time to make sure I'm giving them enough burn-in time.

BTW, the SQ 7062's are impressive in my system but only with the G ST's. Otherwise, they sound good but nothing special. I have hopes for either the RCA or GE drivers to become special with most of my 6V6's, but we'll see.

I'm guessing I will end up with different synergistic couples among 6V6's and drivers. For example, Brimar 12AT7 + Reflektor 6V6; SQ 7062 + G ST; hopefully RCA 12AU7 + Tung-Sol 6V6, and so on.

The Phillips SQ 7062 (SQ stands for Special Quality) that I have come from Brent Jessee.

If the G ST and the 350 B provide similar sound quality, then I would stick to the G ST as it is less demanding on the amp in the long run (350 B is almost a 6L6, with higher current requirements than the 6V6).

I've completed (just about) the burn-in of my new RCA 12AU7 Black Plates, and after careful listening I have several observations for anyone who may care to try these:

1. Best low-mids and base of all the drivers I've tried so far. 2. Excellent midrange. Smooth, natural, thick, neutral.3. Slightly recessed in the high frequencies. Is like applying a treble cut-off. This minor roll-off helps in high pitch passages but I feel in general it misses some important information (textures, limits) in just that top frequency band.

All in all I prefer the SQ 7062's for the mid and high frequencies and the RCA for the mid and low frequencies.

The middle bands are different with each type, but both offer satisfaction. The RCA is smoother and rich with body, the 7062's are more open, airy, fast and detail-retrieving, but less smooth.

Depending on the recording characteristics, one would be better than the other. Both are very good options.

I expect the area inside the red frame to be the most rewarding (to my tastes), and so the possible number of available tube alternatives can be narrowed down when one knows what to look for while keeping the rest of the tube complement fixed.

I haven't finished evaluating the GE 6414 yet, but I'm getting close. So far, it is brighter and edgier than the RCA 12AU7 BP it replaced recently, but part of that may be insufficient 'cooking'.

Update: Today the GE 6414's seemed ready to be pushed, and so I tested them carefully ... No more brightness, still a little edgy in the highs.

It comes second to both the Phillips SQ 7062 and the RCA 12AU7 BP, depending on the recording. Not a bad tube at all though. It is open and pretty neutral, but lags behind the 7062 in texture and the 12AU7 in smoothness and warmth. Highs do not extend as much as with the 7062 but goes higher than the 12AU7.

All in all, a good general purpose driver alternative in the Mini with the described setup.

The warmest 6V6 I know is the Russian Reflektor Plant 6V6, Black Glass. In fact, it needs a commanding driver with relatively high gain to achieve the best it can be, and become the warmest sounding yet still detail retrieving tube it is known for.

In my case, I pair it with Brimar 12AT7, an otherwise edgy and powerful sounding driver that needs a hard/dark output tube to excel. Other 6V6's tend to overdo the highs and to 'ring' when paired with these, especially in high-pitched passages. Only exception being the Big Boys, which can take almost anything and keep their composure.

As far as the Gain-based model chart, it is a rough simplification of what I expect the big picture to be. I don't have the required data (i.e., grading of each 6V6 vs driver combination as per perceived tone), but with the limited sampling I've experienced, I'm definitely guessing this is the general way it can be conceived.

Of course, the curves are most probably exponential (convex in this case) and not linear, but without data the straight line will suffice for a ball park assessment, which is the general object here. For example, I've noticed high sensitivity to gain changes in lower gain tubes and more robust behavior on high gain drivers facing gain changes (typical exponential behavior).

The family of 6V6's depicted in the chart are assumptions stemming from the expected differences in design and components' quality across 6V6's, not real curves. The only true one (as per my results so far) is with the Big Boys.

What's important though, is that I expect the general relationship to remain the same across all 6V6's as the way my setup would react as a function of Gain and Perceived Tone (i.e., some 6V6's would be relatively more robust in their response, while others would react faster to those changes), but the general slope of the curve would stay the same.

For example, I just finished evaluating the GE 6414 and though it belongs to the red framed group in the chart, given its lower relative gain (42 Mu) vis á vis the SQ 7062 (48 Mu) it is not such a surprise I tend to like the latter more.

This is still a rough draft, but I think I'm heading in the right direction, just need to get more samples. This rough model will never be statistically significant as per the myriad of designs and quality components possible out there. This is no problem, as we do not intend other than to narrow the alternatives and increase the probabilities of nailing it sooner than later in a trial-and-error sequence.

Of course, there are differences in drivers too, I suppose, not only gain-derived, but so far the 12AY7 family seems to be predictably at the eye of the hurricane in my setup.

As a reminder, technically this only applies with the set of tubes described, in my Mini Torii, under my listening conditions. Yet, I suspect the model is way more robust than that, though.